Of course you can learn on the web - you are right now. But you probably want to know if you can learn in the formal sense. Can I get a worthwhile degree online? Can I really "to to college" on the web?
You certainly can! Many people do. Many take courses from older brick and mortar "name" schools, such as those listed on the Electronic Campus site, or the Peterson's site, which lists both traditional and non-traditional schools. Many states have built consortia to deliver degree programs - try the University of Texas TeleCampus or the Western Governor's University. Of course there are the non-traditional (but still fully accredited schools, such as Phoenix, Walden, and Capella.
Why? Most likely you are a working adult with a job, mortgage, and family - and you just don't have time to go to English 101 at 1:30 PM each Monday, Wednesday, and Friday. Besides, you may want a Master's degree in Medieval French Literature, but your local university doesn't offer such a degree.
If you think it is some kind of electronic correspondence course, you're wrong. A good online course will probably be more interactive than a traditional classroom. You'll discuss a lot of topics with classmates and professor through discussion board and other tools. You just won't see their faces.
I make my living working with web-based learning. I have taught online (still do on occassion), and assist others in learning how to teach online.
I love it!